v2.0 BREAKING Support change UTXOs & final_cet_sig, for API

Released v1.9.5 as v2.0.0 with no modifications.
Includes:

• v1.9.5 Add final_cet_sig, for API

• v1.9.0 Support change UTXOs (index4+5) Breaking!

Extend all child-key functions with index4, in order to be able to support 'change' UTXOs.
This is a BREAKING change.
The following methods are changed: index parameter is replaced by 2 parameters: index4 and index5:

• get_public_key
• get_address
• verify_public_key
• sign_hash_ecdsa
• sign_schnorr_with_nonce
• verify_schnorr